In general, a gamma ray detector (hereinafter, “detector”) in a positron emission computed tomography (PET) apparatus receives, with a photomultiplier, scintillation light (scintillation photons or optical photons) emitted when a gamma ray emitted from a subject is incident on a scintillator and converts the scintillation light into electronic signals.
For a conventional detector in a PET apparatus, for example, a detector consisting of a large number of scintillator crystals arrayed and each sized as a unit at a depth of 4 mm, a width of 4 mm, and a height (thickness) of 20 mm is used. A reflective member is interposed between scintillator crystals and is structured to prevent scintillation light occurring in one scintillation crystal from leaking to the surrounding scintillator crystals (to prevent crosstalk). For this reason, the conventional detector discretely identifies in which scintillator crystals scintillation events occur and identifies coincidence counting and line-of-response (LOR) on the basis of the results of the identifying.
A PET apparatus having a depth-of-interaction (COI) detection function has the same system where a detector discretely identifies scintillation events. Regardless in which position in a scintillator crystal a scintillation event occurs, such a conventional PET apparatus identifies the scintillation event as one occurring in a single scintillator crystal in a finite size. For this reason, the form of expression of the position in which the identified scintillation event occurs serves as the main item that gives a limit on the spatial resolution and the time resolution.